[U-Boot] [PATCH 2/9] arm64: Make full va map code more dynamic

[Stephen Warren]

On 02/23/2016 10:30 AM, Simon Glass wrote:
> Hi Stephen,
>
> On 23 February 2016 at 10:21, Stephen Warren <swarren at wwwdotorg.org> wrote:
>> On 02/23/2016 06:17 AM, Simon Glass wrote:
>>>
>>> Hi Alex,
>>>
>>> On 21 February 2016 at 18:57, Alexander Graf <agraf at suse.de> wrote:
>>>>
>>>> The idea to generate our pages tables from an array of memory ranges
>>>> is very sound. However, instead of hard coding the code to create up
>>>> to 2 levels of 64k granule page tables, we really should just create
>>>> normal 4k page tables that allow us to set caching attributes on 2M
>>>> or 4k level later on.
>>>>
>>>> So this patch moves the full_va mapping code to 4k page size and
>>>> makes it fully flexible to dynamically create as many levels as
>>>> necessary for a map (including dynamic 1G/2M pages). It also adds
>>>> support to dynamically split a large map into smaller ones when
>>>> some code wants to set dcache attributes.
>>>>
>>>> With all this in place, there is very little reason to create your
>>>> own page tables in board specific files.
>>
>>
>>>>    static struct mm_region mem_map[] = CONFIG_SYS_MEM_MAP;
>>>
>>>
>>> I am not ken on the idea of using a big #define table on these boards.
>>> Is there not a device-tree binding for this that we can use? It is
>>> just a data table, We are moving to Kconfig and eventually want to
>>> drop the config files.
>>
>>
>> I would strongly object to making the MMU setup depend on device tree
>> parsing. This is low-level system code that should be handled purely by
>> simple standalone C code.
>
> Because...?

There is literally zero benefit from putting the exact same content into 
DT, and hence having to run significantly more code to parse DT and get 
back exactly the same hard-coded table.

DT is not a goal in-and-of-itself. In some cases there are benefits to 
placing configuration data outside a binary, and in those cases DT is an 
acceptable mechanism to do that. However, any benefit from doing so 
derives from arguments for separating the data out of the code, not 
because "use DT" is itself a benefit.